One of the central issues in biology is the question of how telomeres are involved in genetic stability. Recombinational telomere elongation (RTE) is a mechanism by which cells elongate their telomeres in the absence of telomerase activity. Human ALT (alternative lengthening of telomeres) tumors depend upon RTE to lengthen telomeres and continue cellular division when telomerase is absent, as is the case in most human somatic cells. One of the components involved in telomere maintenance and homeostasis is Mre11, a multifunctional protein whose primary role is the recognition and repair of double-strand breaks. The long-term goal of the proposed research is to determine the mechanism by which cells undergo recombinational telomere elongation (RTE) and to apply this knowledge to the treatment of human cancers, such as osteosarcomas, caused by ALT cells. The specific objective of this project is to more clearly characterize the specific recombinational mechanism responsible for telomere elongation in yeast. These experiments use the novel gain-of-function allele, mre11- A470T, which is capable of inducing an unusual variation of telomeric recombination in telomerase-negative cells. This variation is responsible for an observed bypass of senescence in telomerase-negative, mre11-A470T cells. The central hypothesis is that RTE in yeast is accomplished via the break-induced replication (BIR) pathway and that the mre11-A470T allele induces aberrant telomeric BIR events. This alteration of the BIR pathway may occur by a generalized or specific enhancement of the activity, regulation, or specificity of BIR enzymatic machinery. The rationale behind the proposed experiments is that study of mre11-A470T provides an excellent and unique opportunity to probe the regulation of telomere recombination by Mre11. The significance of this work is that it will provide us with a greater understanding of the regulation, requirements, and characteristics of recombinational telomere elongation and the role of Mre11 in this process. This information will be essential to our understanding and treatment of the cancer progression of human ALT cells.